The present invention is particularly concerned with such devices having a depth sounding or sonar function where radiant energy is transmitted downwardly into the water with echo signals being returned from various reflecting surfaces provided for example by the bottom surface of the water and fish, for example. Depth sounding devices of this type have been disclosed for example in U.S. Pat. No. 3,733,582 issued May 15, 1973 to Eck et al, entitled "Digital Depth Sounder" and U.S. Pat. No. 4,597,069 issued June 24, 1986 to Milano et al, entitled "Depthsounder/Fish Finder". In addition, U.S. Pat. Nos. 3,879,697 and 4,538,249 issued respectively on Apr. 22, 1975 and Aug. 27, 1985 to Richard both disclosed systems employing an ultrasonic Doppler effect for detecting the presence of underwater objects at least partly based upon motion.
The above patents are referenced herein merely to show a variety of different devices for detecting the presence of underwater objects. However, the systems disclosed by the above patents are generally outside of the scope of the present invention. Initially, the present invention is concerned only with devices employing depth sounding or sonar effects rather than Doppler effect systems as disclosed in the latter two patents.
Although the first two patents referred to above do relate to systems employing depth sounding or sonar effects, they do not provide a visual display of the type contemplated by the present invention. Rather, the systems of those two patents produced a digital signal or display which required interpretation by the user in assessing underwater conditions.
A number of prior art systems have been made available which employed depth sounding or sonar effects together with an XY display of a type generally contemplated by the present invention. The earliest of these devices included an XY display in the form of a paper strip chart. In these devices, vertical scans of echo signals were recorded with the vessel in successive locations to produce an XY display corresponding to echo signals from a succession of such scans.
More recently, such devices with paper strip charts have been replaced by electronic devices including microprocessors and either cathode-ray tube (CRT) or liquid crystal display (LCD) for more rapidly and more simply displaying vertical scans of echo signals.
These devices have also included means for measuring nautical distance traveled by a vessel and temperature of the water, for example, and displaying additional information such as nautical speed, distance traveled during an operating period of the device, ambient temperature of the water and depth of the water, for example measured in feet.
Such devices have been found to be very effective for use by fishermen and others, particularly in determining the depth of the water and for detecting the presence of fish in water beneath the vessel.
However, further improvements have been found desirable in such devices in accordance with the present invention. In particular, it was discovered that the user of such devices had difficulty in determining the nautical range or distance of various objects illustrated on the XY display from a base point such as the instant location of the vessel. In the past, it was generally difficult to accurately portray such information. As noted above, the Y axis of the display corresponds to depth in the water while the X axis corresponds to distance from the vessel. However, the distance, more particularly the nautical distance, between the vessel and any point along the X axis of the screen does not remain consistent but tends to vary, for example with varying nautical speed of the vessel and/or the time required for a vertical scan of the echo signal. Accordingly, in the past, users had difficulty in accurately assessing nautical range as portrayed along the X axis of the display.